The present invention relates generally to orthopaedic screws or pins, and in particular to a bone screw having antimicrobial activity.
Infection frequently accompanies the use of external bone fastening or fixation systems having screws or pins (hereafter xe2x80x9cscrewsxe2x80x9d), which are implanted through soft tissue and into bone. For instance, bone screws may entail infection in soft-tissue by introducing microorganisms into an operation wound. Commonly known as pin-tract infection (xe2x80x9cPTIxe2x80x9d), these infections may further affect the bone. Accordingly, medical devices which are implanted through soft tissue have been developed with antibacterial effects to prevent infection.
One bone screw known from the related art is disclosed in Swiss Patent No. 657,519 to Blaettler. Based on the already long known antibacterial effect of silver, the ""519 patent teaches a surgical implant which is silver-coated. The silver coating is not deposited directly on the substrate, rather, it is deposited on an intermediate copper layer. This allows the concentration of silver ions to reach levels (typically about 1.42 xcexcg/ml), which may inhibit osteoblasts, thereby limiting the formation of new bone.
Another medical device known from the related art is disclosed in U.S. Pat. No. 5,759,564 which teaches a catheter comprising an iontophoretic material and a structure for medical devices that reduces infection by killing microorganisms with controlled oligodynamic iontophoresis. Iontophoresis is the movement of ions in a conductive fluid under the influence of low-strength electric fields.
Additionally, U.S. Pat. No. 5,108,399 discloses a resorbable screw or threaded bolt for fixation of a bone plate and teaches a resorbable screw having a synthetic film comprising collodially distributed silver for preventing infection. This screw, however, is not structurally suitable for use with an external fixation device.
Also, U.S. Pat. No. 5,906,600 discloses an antimicrobial plastic tube or synthetic hose having a silver coating. A slit extending over the entire length of the device allows the hose to be radially pushed (or slipped) onto an orthopedic fastening element so that the antimicrobial effect of the hose surround these pieces. The antimicrobial hose, however, increases trauma to the patient because a longer incision is required to insert the surrounded fastening element through the incision and soft tissue near a bone. The antimicrobial hose also incurs the disadvantage of not being capable of being sterilized and reused.
Despite these developments, a need exists for a bone screw having antibacterial activity that is particularly adapted for use in an external fixation system and which precludes infection.
The present invention is directed to a bone pin formed of a biocompatible material and comprising a first portion located at a distal end of the pin and having a threaded section for insertion into bone and a second portion located at a proximal end of the pin. In an exemplary embodiment of the bone pin, the threaded section is at least 3 mm in length and the second portion has a length between 2 cm and 3 cm.
The bone pin also comprises a third portion located between the first and second portions, which is at least partially coated with an antibacterial agent directly deposited on the biocompatible material free of any intermediate layer. The antibacterial agent is non-colloidal silver, and the biocompatible material is selected from a group consisting of steel, titanium, tantalum, and niobium. Alternatively, the biocompatible material may be selected from a group consisting of polylactides, polyurethanes, hydroxyapatites, glasses, ceramics, and carbon fibers.
The silver may be electrochemically deposited on the biocompatible material, or may comprise a foil deposited on the biocompatible material. In general, the antibacterial agent covers at least 10 percent of the bone pin surface area and forms a layer having a thickness of at least about 10 xcexcm. In one embodiment of the present invention, the antibacterial agent forms a plurality of rings spaced axially along the third portion. In another embodiment, the antibacterial agent forms a plurality of strips spaced radially about at least a part of the third portion.
The present invention also relates to a method for external fixation of a bone comprising the steps of threadably inserting the first portion of a bone pin according to the present invention into a bone and applying an electric current to the bone pin to stimulate formation of new bone and increase infection resistance of the antibacterial agent. In one embodiment of the method, the electric current is 0.25 xcexcA. In another embodiment, the electric current is between 5 and 20 xcexcA.
Essentially, the general advantages offered by the present invention include use of the bone screw without a silver coating and in a fully conventional manner; re-use following preparation and sterilization; and acts as a prophylaxis against infection. This prophylactic effect can be significant when there is shortage of drugs, for example, in developing countries.